HHH-9106 :  Merging multiple representations of the same entity (using S...

...trategySelector)

Has the following changes:
* EntityCopyObserver moved to org.hibernate.event.spi (impls are still in org.hibernate.event.internal)
* StrategySelector is added for EntityCopyObserver impls using short names;
* DefaultMergeEventListener.entityCopyObserverStrategy instance field caches the value for hibernate.event.merge.entity_copy_observer, defaulting to EntityCopyNotAllowedObserver.SHORT_NAME;
* EntityCopyAllowedMergeEventListener and JpaEntityCopyAllowedMergeEventListener were deleted.